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Evolution of Enclaves in Beijing Fangshan Granodiorite and Genesis of Dioritic Microgranular Enclaves

TAO Ji-donga,MA Chang-qianb,ZHANG Jin-yangc,WANG Shu-chund,WANG Ren-jingb(a.Graduate School;b.Faculty of Earth Sciences;c.Faculty of Earth Resources;d.Information Center,China University of Geosciences,Wuhan 430074,China)  
According to the statistics of 2615 enclaves in 53 enclave observation spots,enclaves in Fangshan granodiorite are divided into two categories,xenolith enclave and microgranular enclave.The concentric zonal structure of enclave was formed because of the obvious differential concentration of enclave and pluton components,physicochemical imbalance and interaction between enclave and magma.Xenolith enclave zonal structure developed much,especially for carbonate enclaves.According to the observation on more than 50 enclave rock slices and the results of mineral and rock chemical analysis,the evolution law of xenolith enclave is divided into four phases: phase of hornfels,phase of ultramafic basic rock,phase of intermediate-basic rock,and phase of intermediate-acid rock,then till disapearance of enclave.The identification of 56 microgranular enclave rock slices shows that,the main composite minerals are hornblende,biotite and plagioclase and that components and contents are equivalent with diorites,thus microgranular enclave is also called dioritic enclave.The quantitative statistics demonstrates that enclave with more than 35% dark minerals is called dark dioritic enclave,and enclave with less than 35% dark minerals is called light-colored dioritic enclave.Some of the latter consititues the light-colored edge of the former.According to the macro-characteristics and distribution law of dioritic microgranular enclaves,the special texture and structure,the contrast between the main rock-forming minerals of dioritic microgranular enclave and the host rock,as well as the genesis analysis on double crystal type percentage of 6 187 plagioclase,the conclusion is drawn as follows: despite the variety of the genesis types of dioritic microgranular enclave in Fangshan granodiorite,most of the genesis type is probably the result of the xenolith metamorphic transformation.
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